linux-hardening - Re: [PATCH v10 01/15] modpost: fix removing numeric suffixes

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Message-Id: <20220524113337.4128239-1-alexandr.lobakin@intel.com>
Date:   Tue, 24 May 2022 13:33:37 +0200
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        linux-hardening@...r.kernel.org, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
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Subject: Re: [PATCH v10 01/15] modpost: fix removing numeric suffixes

From: Masahiro Yamada <masahiroy@...nel.org>
Date: Tue, 24 May 2022 03:04:00 +0900

> On Thu, Feb 10, 2022 at 3:59 AM Alexander Lobakin
> <alexandr.lobakin@...el.com> wrote:
> >
> > `-z unique-symbol` linker flag which is planned to use with FG-KASLR
> > to simplify livepatching (hopefully globally later on) triggers the
> > following:
> >
> > ERROR: modpost: "param_set_uint.0" [vmlinux] is a static EXPORT_SYMBOL
> >
> > The reason is that for now the condition from remove_dot():
> >
> > if (m && (s[n + m] == '.' || s[n + m] == 0))
> >
> > which was designed to test if it's a dot or a '\0' after the suffix
> > is never satisfied.
> > This is due to that `s[n + m]` always points to the last digit of a
> > numeric suffix, not on the symbol next to it (from a custom debug
> > print added to modpost):
> >
> > param_set_uint.0, s[n + m] is '0', s[n + m + 1] is '\0'
> >
> > So it's off-by-one and was like that since 2014.
> > Fix this for the sake of upcoming features, but don't bother
> > stable-backporting, as it's well hidden -- apart from that LD flag,
> > can be triggered only by GCC LTO which never landed upstream.
> >
> > Fixes: fcd38ed0ff26 ("scripts: modpost: fix compilation warning")
> > Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> > ---
> >  scripts/mod/modpost.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > index 6bfa33217914..4648b7afe5cc 100644
> > --- a/scripts/mod/modpost.c
> > +++ b/scripts/mod/modpost.c
> > @@ -1986,7 +1986,7 @@ static char *remove_dot(char *s)
> >
> >         if (n && s[n]) {
> >                 size_t m = strspn(s + n + 1, "0123456789");
> > -               if (m && (s[n + m] == '.' || s[n + m] == 0))
> > +               if (m && (s[n + m + 1] == '.' || s[n + m + 1] == 0))
> >                         s[n] = 0;
> >
> >                 /* strip trailing .lto */
> > --
> > 2.34.1
> >
> 
> This trivial patch has not been picked up yet.
> 
> I can apply this to my tree, if you want.

It's a good idea, I'd like to!
I don't use `-z unique-symbol` for FG-KALSR anymore*, but this fix
is not directly related to it and can be taken independently.
Should I change the commit message or it's ok to take it as it is?

> 
> Please let me know your thoughts.
> 
> 
> -- 
> Best Regards
> Masahiro Yamada

* I'm planning to submit a new rev of FG-KASLR series soon, but
since I'm too busy with XDP for now, it will happen no sooner than
in a couple months =\

Thanks!
Al